Release on demand corrosion inhibitor composition

ABSTRACT

Disclosed is cross-linking process for cross-linking polymeric chains in a coating composition. In one embodiment the process utilizes a Hantzsch dihydropyridine reaction to form reaction products including cross-linking polymeric resin chains having beta-keto ester functions using a source of aldehyde and a source of ammonia or a primary amine to form a permanent dihydropyridine bond between the beta-keto ester functions. The novel cross-linking reaction can occur at lower temperatures compared to typical cross-linking reactions and can occur in aqueous solutions that have a neutral to mildly alkaline pH of from 6 to 11. The novel cross-linking reaction provides many advantages to performing cross-linking of polymeric chains in coating resins.

RELATED APPLICATIONS

This U.S. National Stage patent application claims the benefit ofInternational Application No. PCT/US2009/044457 filed May 19, 2009,which claims the benefit of U.S. Provisional Application No. 61/054,360filed on May 19, 2008, the entire disclosures of the applications beingconsidered part of the disclosure of this application, and herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

NONE

TECHNICAL FIELD

This invention relates generally to cross-linking reactions in organiccoating resins and, more particularly, to cross-linking reactions basedon the Hantzsch dihydropyridine synthesis reaction.

BACKGROUND OF THE INVENTION

Cross-linking reactions for cross-linking coating resins are known inthe art. Cross-linking can enhance the properties of coated substrates.These properties include mechanical, physical, aesthetic, and corrosionresistance. One difficulty with current cross-linking reactions is thatthey often require high processing temperatures which limits theirusefulness for certain substrates. In addition, many of thecross-linking reactions require non-aqueous solutions or are reversible,which presents performance problems.

It is desirable to develop a cross-linking process and system that canbe utilized at lower reaction temperatures. In addition, it would bedesirable to develop cross-linking processes that can occur in aqueoussystems and that are irreversible.

SUMMARY OF THE INVENTION

In general terms, this invention provides a cross-linking reactionprocess and system that operates at lower temperatures in aqueoussystems and that is irreversible. The process relies on a method forformation of Hantzsch Dihydropyridines as the cross-linking group. Inone embodiment the system uses two equivalents of beta-keto esters onthe resin, such as those provided by acetoacetoxyethyl methacrylate, oneequivalent of aldehyde and one equivalent of ammonia or a primary amineto produce a Hantzsch Dihydropyridine.

These and other features and advantages of this invention will becomemore apparent to those skilled in the art from the detailed descriptionof a preferred embodiment. The drawings that accompany the detaileddescription are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the proposed Hantzsch Dihydropyridinecross-linking mechanism according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention is directed toward a cross-linking mechanism forcross-linking adjacent polymer chains in a coating resin. Cross-linkingreactions have been utilized in the past for coating resins, howeverthey suffer from a need for high heat of reaction, need for harshconditions, non-aqueous systems, or reversibility. The present inventionprovides for a cross-linking reaction that operates at low temperaturesrelative to other cross-linking reactions, occurs in aqueous solutions,and is irreversible. The present cross-linking reaction improves thechemical properties, mechanical properties, barrier properties, physicalproperties and corrosion resistance of the coating resin. The presentreaction can be used to cause cross-linking in thin organic coatings,organic coatings, passivate rinses, sealing rinses, adhesives, andsealants, by way of example. The present invention is directed towardtreatment of bare metal surfaces meaning that the metal surface has notbeen pre-treated with any metal phosphate solutions, chrome-containingrinses, or any other passivating treatments. Metal surfaces that benefitfrom the process of the present invention include steel, cold rolledsteel, hot rolled steel, stainless steel, aluminum, steel coated withzinc metal or zinc alloys such as electrogalvanized steel, Galvalume®,galvanneal, and hot-dipped galvanized steel.

Preferably, the metal surface has been cleaned and degreased prior totreatment according to the present invention. Cleaning of metal surfacesis well known in the art and can include mild or strongly alkalinecleaners. Examples of two alkaline cleaners include Parco® Cleaner ZX-1and Parco® Cleaner 315 both available from Henkel Surface Technologies.Following cleaning the surface is preferably rinsed with water prior totreatment according to the present invention.

The proposed reaction scheme for the present invention is shown inFIG. 1. The Hantzsch dihydropyridine/pyridine synthesis reaction allowsthe preparation of a dihydropyridine derivative by a condensationreaction of an aldehyde with two equivalents of a beta-keto ester, suchas provided by the pendant chains after incorporation ofacetoacetoxyethyl methacrylate into the resin, in the presence ofammonia or a primary amine. A subsequent oxidation or dehydrogenationleads to a pyridine 3,5-dicarboxylate.

Ammonia or a variety of primary amines are useful in present invention.Examples include ammonia, amino acids, diamines, and other moleculeswith primary amine groups. It has been found that the amine levels canbe very high, far in excess of the number of beta-keto ester groups onthe resin without interfering with the Hantzsch Dihydropyridinereaction.

A variety of aldehydes may be used in the present invention including,only by way of example, formaldehyde, salicylaldehyde, cinnamaldehyde,glucose, vanillin, glyoxal, and glyoxylic acid. Preferably the amount ofaldehyde is closely tailored to the level of resin beta-keto estersites. Preferably, the cross-linking solution includes from 0.1 to 1.5equivalents of aldehyde per 2 equivalents of beta-keto ester groups onthe resin, more preferably from 0.5 to 1.1 aldehyde equivalents per 2equivalents of beta-keto ester groups on the resin.

A series of resins were prepared having as the pendent chains withbeta-keto ester functions acetoacetoxyethyl methacrylate (AAEM). This isjust one of the possible pendant chains that could be used in theprocess of the present invention.

Coating Resin Example 1 (3272-096)

An organic coating resin was prepared as described below, it isdesignated as resin 3272-096. The resin includes as monomers:acetoacetoxyethyl methacrylate (AAEM), n-butyl methacrylate, styrene,methyl methacrylate, 2-ethylhexyl acrylate, and ADD APT PolySurf HPwhich is a mixture of methacrylated mono and di-phosphate ester. Anotherpotential source for phosphates in all resins prepared according to thepresent invention is Ebecryl 168 from Radcure Corporation. The totalmonomer distribution in the resin was as follows: 20.00% AAEM, 12.50%n-butyl methacrylate, 15.00% styrene, 27.50% methyl methacrylate, 20.00%2-ethylhexyl acrylate, and 5.00% ADD APT PolySurf HP. The resinpolymerization reaction was run under N₂ with stirring and a heat setpoint of 80° C. The initial charge to the reaction vessel was 241.10grams of deionized (DI) water, 2.62 grams of ammonium lauryl sulfate(Rhodapon L-22 EP), and 2.39 grams of ferrous sulfate 0.5% FeSO₄7H₂O (3ppm). This initial charge was put into the reaction vessel at time zeroand heating to the set point was begun. After 30 minutes a reactor seedcomprising a combination of 5.73 grams of DI water, 0.90 grams ofnon-ionic surfactant Tergitol 15-S-20, 0.13 grams of Rhodapon L-22 EP,2.15 grams of n-butyl methacrylate, 2.57 grams of styrene, 4.74 grams ofmethyl methacrylate, 3.48 grams of 2-ethylhexyl acrylate, 3.41 grams ofacetoacetoxyethyl methacrylate (AAEM), and 0.85 grams of ADD APTPolySurf HP was added to the reaction vessel and heating to the setpoint was continued for another 15 minutes. Then an initial initiatorcharge was added to the vessel comprising 0.32 grams of HOCH₂SO₂Na, 4.68grams of DI water, 0.45 grams of tert-butylhydroperoxide, and anadditional 4.54 grains of DI water and the temperature was maintained atthe set point for another 30 minutes. Then the monomer and initiatorco-feeds were added to the vessel over a three hour period with thetemperature maintained at the set point. The monomer co-feed was 106.92grams of DI water, 17.10 grams of Tergitol 15-S-20, 2.49 grams ofRhodapon L-22 EP, 40.89 grams of n-butyl methacrylate, 48.83 grams ofstyrene, 89.97 grams of methyl methacrylate, 66.10 grams of 2-ethylhexylacrylate, 64.77 grams of AAEM, and 16.19 grains of ADD APT PolySurf HP.The initiator co-feed was 0.97 grams of HOCH₂SO₂Na, 14.03 grams of DIwater, 1.39 grams of tert-butylhydroperoxide, and an additional 13.61grains of DI water. After the three hours a chaser charge was added tothe vessel over a 30 minute period. The chaser charge was 0.32 grams ofHOCH₂SO₂Na, 4.88 grams of DI water, 0.46 grams oftert-butylhydroperoxide, and an additional 4.54 grams of DI water. Thevessel was then held at the set point for one hour and 30 minutes. Thenthe cool down from the set point was begun and continued for 2 hoursuntil the temperature was 38° C. Then the buffer co-feed was added tothe vessel. The buffer co-feed was 5.19 grams of ammonium hydroxide(28%) and 18.48 grams of DI water. Additional non-ionic surfactantstabilizers that could be used in place of Tergitol 15-S-20, which is asecondary alcohol ethoxylate, in this resin and all resins in accordancewith the present invention are other non-ionic stabilizers having ahydrophilic lipophilic balance of from 15 to 18. Examples of thesestabilizers include: other secondary alcohol ethoxylates such asTergitol 15-S-15; blends of ethoxylates such as Abex 2515; alkylpolyglycol ether such as Emulsogen LCN 118 or 258; tallow fatty alcoholethoxylate such as Genapol T 200 and T 250; isotridecyl alcoholethoxylates such as Genapol X 158 and X 250; tridecyl alcoholethoxylates such as Rhodasurf BC-840; and oleyl alcohol ethoxylates suchas Rhoadsurf ON-877.

Coating Resin Example 2 (3272-103)

An organic coating resin was prepared as described below, it isdesignated as resin 3272-103. The resin includes as monomers:acetoacetoxyethyl methacrylate (AAEM), n-butyl methacrylate, styrene,methyl methacrylate, 2-ethylhexyl acrylate, and ADD APT PolySurf HPwhich is a mixture of methacrylated mono and di-phosphate ester. Thetotal monomer distribution in the resin was as follows: 20.00% AAEM,12.50% n-butyl methacrylate, 15.00% styrene, 27.50% methyl methacrylate,20.00% 2-ethylhexyl acrylate, and 5.00% ADD APT PolySurf HP. The resinpolymerization reaction was run under N₂ with stirring and a heat setpoint of 80° C. The initial charge to the reaction vessel was 286.10grams of DI water, 2.47 grams of Rhodapon L-22 EP. This initial chargewas put into the reaction vessel at time zero and heating to the setpoint was begun. After 30 minutes a reactor seed comprising acombination of 5.44 grams of DI water, 0.85 grams of Tergitol 15-S-20,0.12 grams of Rhodapon L-22 EP, 2.04 grams of n-butyl methacrylate, 2.44grams of styrene, 4.49 grams of methyl methacrylate, 3.30 grams of2-ethylhexyl acrylate, 3.24 grams of acetoacetoxyethyl methacrylate(AAEM), and 0.81 grams of ADD APT PolySurf HP was added to the reactionvessel and heating to the set point was continued for another 15minutes. Then an initial initiator charge was added to the vesselcomprising 4.79 grams of DI water and 0.21 grams of (NH₄)₂S₂O₈ and thetemperature was maintained at 80° C. for another 30 minutes. Then themonomer and initiator co-feeds were added to the vessel over a threehour period with the temperature maintained at the set point. Themonomer co-feed was 103.36 grams of DI water, 16.15 grams of Tergitol15-S-20, 2.35 grams of Rhodapon L-22 EP, 38.81 grams of n-butylmethacrylate, 46.34 grams of styrene, 85.38 grams of methylmethacrylate, 62.73 grams of 2-ethylhexyl acrylate, 61.47 grams of AAEM,and 15.37 grams of ADD APT PolySurf HP. The initiator co-feed was 14.36grams of DI water and 0.64 grams of (NH₄)₂S₂O₈. After the three hours achaser charge was added to the vessel over a 30 minute period. Thechaser charge was 0.35 grams of ascorbic acid, 4.65 grams of DI water,0.44 grains of tert-butylhydroperoxide, an additional 4.56 grams of DIwater, and 2.39 grams of ferrous sulfate 0.5% FeSO₄7H₂O (3 ppm). Thevessel was then held at the set point for one hour and 30 minutes. Thenthe cool down was begun and continued for 2 hours until the temperaturewas 38° C. Then the buffer co-feed was added to the vessel. The bufferco-feed was 5.88 grams of ammonium hydroxide (28%) and 18.48 grams of DIwater.

Coating Resin Example 3 (3272-056)

An organic coating resin was prepared as described below, it isdesignated as resin 3272-056. The resin includes as monomers:acetoacetoxyethyl methacrylate (AAEM), n-butyl methacrylate, styrene,methyl methacrylate, 2-ethylhexyl acrylate, and ADD APT PolySurf HPwhich is a mixture of methacrylated mono and di-phosphate ester. Thetotal monomer distribution in the resin was as follows: 20.00% AAEM,12.50% n-butyl methacrylate, 15.00% styrene, 27.50% methyl methacrylate,20.00% 2-ethylhexyl acrylate, and 5.00% ADD APT PolySurf HP. The resinpolymerization reaction was run under N₂ with stirring and a heat setpoint of 70° C. The initial reactor charge was 241.01 grams of DI water,and 2.62 grams of Rhodapon L-22 EP (ammonium lauryl sulfate). The secondreactor charge was 2.39 grams of ferrous sulfate 0.5% FeSO₄7H₂O (3 ppm).The two initiator co-feeds were 1.62 grams of HOCH₂SO₂Na in 23.38 gramsof DI water and 2.31 grams of tert-butylhydroperoxide in 22.69 grams ofDI water. The monomer co-feed was 114.41 grams of DI water, 18.00 gramsof Tergitol 15-S-20 (secondary alcohol ethoxylate surfactant), 2.62grams of Rhodapon L-22 EP, 68.18 grams of AAEM, 43.05 grams of n-butylmethacrylate, 51.39 grams of styrene, 94.70 grams of methylmethacrylate, 69.58 grams of 2-ethylhexyl acrylate, and 17.05 grams ofADD APT PolySurf HP. The neutralizer charge was 6.52 grams of 28%ammonium hydroxide in 18.48 grams of DI water. The process commencedwith adding the initial reactor charge to the reaction vessel withstirring for 30 minutes. Then 25 grams of the monomer co-feed was addedto the reaction vessel as a seed along with 4 milliliters of eachinitiator co-feed and the second reactor charge. Then the monomerco-feed was fed into the reaction vessel over a 3 hour period and theinitiator co-feeds were fed into the reaction vessel over a 4 hourperiod. After the final addition of the initiator co-feeds the reactionwas run for an additional 40 minutes and then cool down to 38° C. wasbegun. After 1 hour and 45 minutes of cool down the neutralizer chargewas added to the reaction vessel.

In a first test two coating compositions were prepared to determine ifthe Hantzsch dihydropyridine reaction could take place in a resincoating composition. The test involved use of resin 3272-096 and eithervanillin or salicylaldehyde as the aldehyde source for the reaction. Therequired amine for the reaction is provided by the Bacote 20® which hasa great excess of ammonium to serve as the amine in the reaction. Bacote20® is one preferred source of ammonium zirconyl carbonate and isavailable from MEI in Flemington, N.J. It provides approximately 20% w/wof ZrO₂. The two test formulas are provided in Table 1 below. Theformula with vanillin did not result in any visual indication of areaction, however, the formula with salicylaldehyde began turning yellowin color indicating a reaction was occurring that resulted in formationof a carbon to nitrogen bond. These results suggest that a Hantzschdihydropyridine reaction is possible in the formulas and that theidentity of the aldehyde source may influence the reaction.

TABLE 1 Component Coating 1 Coating 2 DI water 65.5 65.5 ammoniumzirconyl 24.0 24.0 carbonate (Bacote 20 ®) Resin 3272-096 10.0 10.0vanillin 0.5 salicylaldehyde 0.5

In the next series of experiments a series of coating compositions werecreated using the coating resins described above. The coatingcompositions were prepared as described below in Table 2 and Table 3with the amounts of each component given in weight percentage. In thecoating solutions the source of the aldehyde was varied, amine for theHantzsch dihydropyridine reaction is provide by the excess ammonia inthe Bacote 20®. In addition, amine can be provided by the cysteine.Optionally, the coatings of the present invention can also includeprocessing aids such as waxes which aid in formability of the coatedsubstrates.

TABLE 2 Component 9A 9B 9C 9D 9E 9F 9G DI Water 65.00 64.74 64.72 64.6264.68 64.69 64.69 Bacote 20 ® 24.00 24.00 24.00 24.00 24.00 24.00 24.00V₂O₅ 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Cysteine 0.50 0.50 0.50 0.500.50 0.50 0.50 3272-096 10.00 10.00 10.00 10.00 10.00 10.00 10.00Salicylaldehyde 0.26 Cinnamaldehyde 0.28 Glucose 0.38 Vanillin 0.32 40%Glyoxal 0.31 50% Glyoxylic 0.31 acid

TABLE 3 Component 9H 9I 9J 9K 9L 9M 9N DI Water 65.00 64.76 64.74 64.6464.70 64.71 64.71 Bacote 20 ® 24.00 24.00 24.00 24.00 24.00 24.00 24.00V₂O₅ 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Cysteine 0.50 0.50 0.50 0.500.50 0.50 0.50 3272-103 10.00 10.00 10.00 10.00 10.00 10.00 10.00Salicylaldehyde 0.24 Cinnamaldehyde 0.26 Glucose 0.36 Vanillin 0.30 40%Glyoxal 0.29 50% Glyoxylic 0.29 acid

The prepared coating compositions described in Table 2 and Table 3 werethen coated onto a series of hot dip galvanized (HDG) panels ACT HDG APR31893 for testing of the corrosion resistance of the coatings in neutralsalt spray (NSS) testing using ASTM B117. The test panels were coatedwith the formulas from Tables 2 and Table 3 in a dry in place process asknown to those of skill in the art. The coatings were applied at acoating weight of approximately 200 milligrams per square foot (200milligrams per 929.03 square centimeters) to each panel and then driedto a peak metal temperature of either 200° F. (93° C.) or 300° F. (149°C.) and either put directly into the NSS test or first washed with thealkaline cleaner PCl 338 and then put in to the NSS test. A drop in NSSresults after pre-treatment with PCl 338 would indicate that thecoatings are not alkaline resistant. The coated panels were then testedfor corrosion resistance using NSS according to ASTM B117. For each timepoint multiples of each condition were examined and the percentage ofthe total surface corroded was determined, averaged and reported below.

The results in Table 4 are from panels that were dried to a peak metaltemperature of 200° F. (93° C.) with no treatment with PCl 338. Thecontrols, formulas 9A and 9H, did not include any exogenously addedaldehyde for the Hantzsch dihydropyridine reaction. The best resultswere seen with glucose as the aldehyde source in formula 9D which showedcorrosion resistance that was much higher than the other formulas. Goodresults were also seen in formulas 9J and 9K.

TABLE 4 Formula 24 hr 48 hr 72 hr 96 hr 168 hr 336 hr 504 hr 9A 0 3.78.7 11.3 50 9B 1 34.3 72 76.7 93.3 9C 0 7 10 25.3 50 9D 0 1 1 3 7 33.380 9E 0 7 10 30 60 9F 0 7 7 16 70 9G 0 5.3 6.7 17.3 39 60 90 9H 0 1 14.3 33.3 76.7 9I 0 30 50 9J 0 0.3 1.3 3 19 9K 0 1 1 5.3 24.3 70 9L 0 3 915.7 38.7 93.3 9M 0 25 30 40 50 9N 0 4.3 7 18.7 60

The results in Table 5 are from panels that were dried to a peak metaltemperature of 300° F. (149° C.) with no pre-treatment with PCl 338. Thecontrols, formulas 9A and 9H, did not include any exogenously addedaldehyde for the Hantzsch dihydropyridine reaction. The results showthat for virtually all formulas increasing the PMT to 300° F. (149° C.)resulted in worse performance in the NSS test or no improvement. Atthese higher PMT conditions the best corrosion resistance was seen informulas 9D and 9G.

TABLE 5 Formula 24 hr 48 hr 72 hr 96 hr 168 hr 336 hr 9A 0 0 0 1.7 50 9B50 9C 0 7 11 30 50 9D 0 3 3 4.3 43.3 9E 0 20.7 50 9F 0 6 50 9G 0 9.7 6.711.7 36.7 60 9H 0 1 18.7 40 65.3 93.3 9I 0 50 9J 0 10 30 40 50 9K 0 3050 9L 0 20 50 9M 0 50 9N 0 10 10 16 50

The results in Table 6 are from panels that were dried to a peak metaltemperature of 200° F. (93° C.) followed by pre-treatment with PCl 338prior to the NSS test. The controls, formulas 9A and 9H, did not includeany exogenously added aldehyde for the Hantzsch dihydropyridinereaction. The results for the formulas 9A to 9N all were worse after thePCl 338 pre-treatment indicating that the coatings are not alkalineresistant,

TABLE 6 Formula 24 hr 48 hr 72 hr 96 hr 168 hr 9A 7 50 50 50 50 9B 30 509C 3 50 9D 5 50 9E 3 50 9F 3 50 9G 0 3 10 50 9H 3 30 50 9I 7 50 50 9J 07 16 25 50 9K 0 20 50 9L 3 30 50 9M 20 50 9N 0 40

The results in Table 7 are from panels that were dried to a peak metaltemperature of 300° F. (149° C.) followed by pre-treatment with PCl 338prior to the NSS test. The controls, formulas 9A and 9H, did not includeany exogenously added aldehyde for the Hantzsch dihydropyridinereaction. For the formulas 9A to 9N the effect of PCl 338 pre-treatmentwas to generally reduce the corrosion resistance. Some formulas showedno change, but most were negatively effected by the pre-treatment.

TABLE 7 Formula 24 hr 48 hr 72 hr 96 hr 168 hr 9A 3 10 80 9B 50 50 9C 350 9D 3 60 9E 10 50 9F 7 20 80 9G 0 16 50 9H 0 20 50 9I 16 50 9J 1 30 5040 50 9K 0 30 50 9L 10 50 50 9M 7 50 9N 0 30 50

In another series of experiments these formulas and peak metaltemperatures were tested on HDG panels to determine their solventresistance to methylethyl ketone (MEK). The test was performed as knownto those in the art. Briefly, coated test panels were rubbed by machinewith a pad coated in the MEK for set period of time and then graded forappearance. The results are presented in Table 8 below, with a level 1being the worst appearance and 10 being the best. Control panels forformula 9N were prepared using acetic, acid, which provides no aldehydefunction, in place of the 50% glyoxylic acid at a PMT of 200° F. (93°C.). This control produced an appearance level of 5. The formulas 9A to9N seemed to benefit from the higher PMT of 300° F. (149° C.) almostacross the board. Again best results were seen with eithersalicylaldehyde, cinnamaldehyde, glucose, or glyoxylic acid.

TABLE 8 HGD PMT 200° F. HDG 300° F. Formula (93° C.) (149° C.) 9A 6 8 9B4 8 9C 9 9 9D 5 8 9E 4 5 9F 8 9 9G 8 10 9H 9 9 9I 9 7 9J 8 9 9K 9 9 9L 59 9M 7 7 9N 9 9

In a final series of experiments the effect of PMT on alkalineresistance of the coatings was determined. For this experiment panelswere created using the following coating formula: 65.00% DI water,24.00% Bacote 20, 0.50% V₂O₅, 10.00% resin 3272-056, and 0.5% Cysteine.In this coating the aldehyde is coming from the sodium formaldehydesulfoxalate initiator used to form the resin 3272-056. The coated panelswere coated with approximately 200 milligrams per square foot (200milligrams per 929.03 square centimeters) and then dried to PMT of 200,250, 300, or 350° F. (93, 121, 149, or 177° C.). The alkaline treatmentwas as follows: a portion of each panel was dipped into a 20% NaOHsolution for approximately 1 to 2 minutes and then removed for visualevaluation. The results showed that using PMT of 200 or 250° F. (93 or121° C.) provided no resistance to alkaline dip, in fact, the dipremoved the entire coating down to bare metal. By way of contrast PMT of300 or 350° F. (149 or 177° C.) provided complete resistance to thealkaline treatment. This indicates that the Hantzsch Dihydropyridinereaction may require PMT of from 250 to 300° F. (121 to 149° C.) forcompletion using this formulation.

Coatings prepared according to the present invention are designed to beapplied directly to bare metal substrates without the need for anyphosphate or other pre-treatments other than cleaning. They can beapplied at any desired coating weight required by the situation,preferably they are applied at a coating weight of from 150 to 400milligrams per square foot (150 to 400 milligrams per 929.03 squarecentimeters), more preferably at from 175 to 300 milligrams per squarefoot (175 to 300 milligrams per 929.03 square centimeters) and mostpreferably at from 175 to 250 milligrams per square foot (175 to 250milligrams per 929.03 square centimeters). The coatings of the presentinvention are dry in place coatings as known in the art and arepreferably dried to a peak metal temperature of from 180 to 350° F. (82to 177° C.), more preferably to a PMT of from 200 to 325° F. (93 to 163°C.).

The foregoing invention has been described in accordance with therelevant legal standards, thus the description is exemplary rather thanlimiting in nature. Variations and modifications to the disclosedembodiment may become apparent to those skilled in the art and do comewithin the scope of the invention. Accordingly, the scope of legalprotection afforded this invention can only be determined by studyingthe following claims.

We claim:
 1. An aqueous coating composition comprising: reactionproducts of a polymeric resin containing pendant beta-keto esterfunctions with an aldehyde and a source of either ammonia or a primaryamine; wherein said polymeric resin comprises a plurality of polymericchains and wherein each of said polymeric chains contains at least oneof said pendant beta-keto ester functions; wherein said aldehyde isselected from the group consisting of formaldehyde, salicylaldehyde,cinnamaldehyde, vanillin, glyoxal, glyoxylic acid, and mixtures thereof;and wherein a portion of said polymeric chains are cross-linked to eachother by a Hantzsch dihydropyridine reaction between two equivalents ofsaid beta-keto ester functions, each of said beta-keto ester functionson a different polymeric chain, one equivalent of said aldehyde and oneequivalent of said ammonia or said primary amine, thereby forming aplurality of polymeric chains each crosslinked to another polymericchain by a Hantzsch dihydropyridine reaction.
 2. The aqueous coatingcomposition as recited in claim 1, further comprising a wax.
 3. Theaqueous coating composition as recited in claim 1, comprising from 0.1to 1.5 equivalents of said aldehyde groups per each 2 equivalents ofbeta-keto ester functions on said resin.
 4. The aqueous coatingcomposition as recited in claim 1, wherein said beta-keto esterfunctions are provided by incorporation of acetoacetoxyethylmethacrylate into said polymeric resin.
 5. The aqueous coatingcomposition as recited in claim 1, wherein said aldehyde furthercomprises.
 6. The aqueous coating composition as recited in claim 1,wherein said source of primary amine comprises an amino acid.
 7. Theaqueous coating composition as recited in claim 6, wherein said aminoacid comprises cysteine.
 8. The aqueous coating composition as recitedin claim 7, wherein said cysteine is present in an amount of 0.5% byweight.
 9. The aqueous coating composition as recited in claim 1,wherein said coating composition has a pH of from 6 to
 11. 10. Theaqueous coating composition as recited in claim 1, wherein said sourceof ammonia comprises ammonium zirconyl carbonate.
 11. The aqueouscoating composition as recited in claim 1, wherein said coatingcomposition further comprises vanadium pentoxide present in an amount of0.5% by weight.
 12. A method of coating a metal substrate comprising thesteps of: a) providing a metal substrate; b) providing an aqueouscoating composition comprising reaction products of a polymeric resincontaining pendant beta-keto ester functions with an aldehyde and asource of either ammonia or a primary amine; wherein said polymericresin comprises a plurality of polymeric chains and wherein each of saidpolymeric chains contains at least one of said pendant beta-keto esterfunctions; wherein said aldehyde is selected from the group consistingof formaldehyde, salicylaldehyde, cinnamaldehyde, vanillin, glyoxal,glyoxylic acid, and mixtures thereof; and wherein a portion of saidpolymeric chains are cross-linked to each other by a Hantzschdihydropyridine reaction between two equivalents of said beta-keto esterfunctions, each of said beta-keto ester functions on a differentpolymeric chain, one equivalent of said aldehyde and one equivalent ofsaid ammonia or said primary amine, thereby forming a plurality ofpolymeric chains each crosslinked to another polymeric chain by aHantzsch dihydropyridine reaction; and c) applying the aqueous coatingcomposition to the metal substrate and drying the aqueous coatingcomposition in place on the metal substrate.
 13. The method as recitedin claim 12, wherein step c) comprises drying the aqueous coatingcomposition at a peak metal temperature sufficient for the Hantzschdihydropyridine reaction to proceed to sufficiently cross-link thepolymeric chains to achieve a coating that is resistant to removal fromthe metal substrate by exposure to a 20% NaOH solution for 1 minute. 14.The method as recited in claim 12, wherein step c) comprises drying theaqueous coating composition at a peak metal temperature of from 82 to177° C.